Amino Acids

The isoelectric point is the pH at which an amino acid carries zero net charge, and its value is set by the balance of all ionisable groups present. In glycine, only the alpha-carboxyl and alpha-amino groups ionise, and the zero-charge condition is met near pH 6. An acidic amino acid such as aspartic or glutamic acid carries an additional carboxyl group on its side chain; this extra acidic group tends to lose a proton and acquire negative charge, so a more acidic environment (a lower pH with higher hydrogen-ion concentration) is required to suppress that ionisation and bring the net charge to zero. Consequently the isoelectric point is shifted to a lower pH than glycine, making the first option correct. The claim that an extra carboxyl adds basic character is chemically backwards, since carboxyl groups are acidic, so the higher-pH option is wrong. The isoelectric point cannot be identical to glycine because the group balance has changed, and three ionisable groups can certainly define a net-zero pH, so it is not undefined. This follows the NCERT idea of acidic and basic amino acids. As a plausibility check, basic amino acids with extra amino groups instead show raised isoelectric points, confirming the trend works in the expected opposite direction.